1. Field of the Invention
This invention relates to an optical recording medium of phase change type wherein a small record mark beyond diffraction limit can be read.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is phase change optical recording medium wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change in reflectance by the crystallographic change is detected for reading of the information. The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit is simple as compared to magnetooptical recording media.
Most optical recording media of phase change type used chalcogenide systems such as Gexe2x80x94Te system and Gexe2x80x94Sbxe2x80x94Te system which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was also recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds have been investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb2 or IIb-IVb-Vb2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228. Among the chalcopyrite compounds, AgInTe2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai Nos. (JP-A) 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992. In addition to the optical recording media of phase change type wherein chalcopyrite compounds are used, optical recording media of phase change type wherein AgSbTe2 phase is formed with the crystallization of the recording layer is disclosed in JP-A 267192/1992, 232779/1992, and 166268/1994.
When information is recorded on the optical recording medium of phase change type, the entire recording layer is first brought into crystalline state, and then, a laser beam of high power (recording power) is applied so that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallization temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallization temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single light beam.
In general, recording density of optical recording media including optical recording media of phase change type can be increased to a level higher than that of magnetic recording media. Today, further increase in the recording density is demanded for processing an enormous amount of information as in the case of image processing. Recording density per unit area can be increased either by reducing the track pitch, or by increasing linear recording density through shortening of the space between the record marks. An excessively high track density or an excessively high linear recording density in relation to beam spot of the reading light beam, however, invites lowering of C/N, and signal reading eventually becomes impossible. Resolution in the signal reading is determined by the diameter of the beam spot, and more illustratively, diffraction limit is generally 2NA/xcex, which is spacial frequency at the reading light beam wavelength of xcex and numerical aperture NA of the optical system in the reading system. Accordingly, use of the reading light beam with a shorter wavelength and increase in NA are effective for increasing the C/N in the reading and for improving the resolution. A number of proposals have been made through many technical investigations, and such proposals present a wide variety of technological challenges.
JP-A 96926/1990 proposes a recording matrix having a layer comprising a nonlinear optical material for realizing super-resolution. The nonlinear optical materials proposed in JP-A 96926/1990 are materials whose optical properties change by irradiation with a radiation, and such change in the optical properties include change in transparency, reflectance, and refractive index as well as change in the configuration of the layer. Reading out of a smaller region is enabled by irradiation of the information-bearing surface with the reading light beam through the layer of such nonlinear optical material.
In JP-A 96926/1990, a breaching layer is proposed as the layer of the nonlinear optical material. This breaching layer exhibits increase in transparency with the increase in the intensity of the incident radiation, and exemplary materials indicated for the breaching layer are GaAs, InAs, and InSb. In the case of the layer comprising such nonlinear optical material, a reading light beam of high energy density is required since all of absorption center should be excited.
JP-A 89511/1993, 109117/1993, and 109119/1993 disclose optical discs comprising a substrate having optically readable phase pits formed thereon and a layer of the material that exhibits reflectance change by temperature. The layer of such material is a layer which functions in a way substantially the same as the nonlinear material layer of JP-A 96926/1990, and this layer is provided for the purpose of realizing a resolution beyond that determined by the wavelength xcex of the reading light and the numerical aperture NA of the objective lens. This layer, however, requires a reading light of high power since change from crystalline to liquid state or from amorphous to liquid state is required in such layer.
JP-A 169094/1995 discloses an optical recording medium wherein a recording layer of phase change type and a mask layer are disposed with an intervening intermediate dielectric layer. This mask layer is a layer which functions in substantially the same way as the nonlinear material layer of JP-A 96926/1990, and comprises a phase change material. In the reading of this optical recording medium, the mask layer is melted to reduce the imaginary part of the complex refractive index in the range of 0.25 to 1.0 to thereby enable reading of a small record mark through the molten part of the mask region.
A further improvement recently proposed in the diffraction limit is use of near field light. Appl. Phys. Lett., Vol.73, No.15, pp.2078-2080, 1998, for example, adopts a structure similar to the super-resolution medium described in JP-A 96926/1990, and in this structure, the distance between the nonlinear optical material layer and the recording layer is reduced to enable use of near field light for improving the diffraction limit. In this report, the nonlinear optical material layer comprising Sb and the recording layer comprising Ge2Sb2Te5 are formed to sandwich a SiN layer of 20 nm thick, and there is stated that such structure has enabled to read out record marks of 100 nm or less. It should be noted that, in the Example of JP-A 169094/1995, the distance between the nonlinear optical material layer (mask layer in JP-A 169094/1995) and the recording layer (i.e. the thickness of the intermediate dielectric layer in JP-A 169094/1995) is 180 nm, and such distance between the nonlinear optical material layer and the recording layer is far greater than that described in Appl. Phys. Lett., Vol.73, No.15, pp.2078-2080, 1998.
The inventors of the present invention produced the medium described in Appl. Phys. Lett., Vol.73, No.15, pp.2078-2080, 1998, and actually read the thus produced medium. In this attempt, readability of the record marks was confirmed. Initial C/N, however, was quite low and the C/N experienced a rapid drop with the signal reading and the medium soon became unreadable. In short, the medium had poor initial properties and impractical reading stability. In the investigation for the reason of such poor stability, it was found that such poor stability was caused by substantial erasing of the record marks after quite short period due to the reading power of relatively high energy level applied to the medium in order to change the optical properties of the nonlinear optical material layer.
In view of the situation as described above, an object of the present invention is to realize a high C/N and to suppress decrease of the C/N in the reading in an optical recording medium wherein reading of small record marks beyond diffraction limit is enabled by forming the layer of nonlinear optical material and the recording layer of phase change type extremely close to each other.
Such objects are attained by the present invention as described in (1) to (3), below.
(1) An optical recording medium having a mask layer and a recording layer of phase change type with an intervening intermediate dielectric layer such that an optical aperture can be formed in the mask layer by irradiation of reading light beam from the side of the mask layer, wherein
the intermediate dielectric layer has a thickness of 5 to 70 nm; and
the recording layer contains Ag, In, Sb and Te as its main components and Ge and/or N as its sub-component.
(2) An optical recording medium having a mask layer and a recording layer of phase change with an intervening intermediate dielectric layer such that an optical aperture can be formed in the mask layer by irradiation of reading light beam from the side of the mask layer, wherein
near field light generated near the optical aperture is used in the reading; and
the recording layer contains Ag, In, Sb and Te as its main components and Ge and/or N as its sub-component.
(3) An optical recording medium of the above (1) or (2) wherein the mask layer has a thickness of 10 to 25 nm.